Ternary semiconductor alloys can be obtained by solidifying a melt or solution of three elements. Generally, this is done by melting appropriate amounts of three elements, typically denoted A, B, and C, together. For example, an amount x of A, a unit amount of B, and an amount 1-x of C, where x is equal to 0 to 1 times the unit amount of B that is present, can be melted together to form the ternary. In the melt or solution, A.sub.x and C.sub.1-x react with B to form two molten binaries (AB).sub.x and (CB).sub.1-x. Solidification of these molten binaries produces an alloy having the formula A.sub.x BC.sub.1-x. Alternatively, the process can be carried out by first mixing two separately prepared binaries having he formulae (AB).sub.x and (CB).sub.1-x and solidifying the mix to produce an alloy having the formula A.sub.x BC.sub.1-x.
In practice, growing ternaries from melts becomes difficult due to wide separation between the solidus-liquidus curves of the pseudo-binary phase diagram. This arises primarily because of differences in lattice constants (sometimes dubbed "lattice mismatch") and melting points of the alloys' constituent binaries. As a result, the dissimilar properties, in particular, dissimilarities in lattice constants, give rise to defects in the alloy crystals. Such defects include dislocations, mechanical cracks, non-uniform composition (for example, due to segregation), inclusions, precipitates, dendrites, and the like. For example, InGaSb in which In makes up more than about 10% cannot be prepared without the occurrence of defects.
To overcome these problems, at present, ternary crystals are produced in the form of thin layers, by a method commonly referred to as epitaxial growth. Typically, this is carried out by non-equilibrium growth techniques from dilute solutions or from a vapor phase on binary substrates by compositionally graded buffers. However, epitaxial growth techniques are expensive, and, when used to grow semiconductor alloys, these techniques increase the cost of the alloy by several fold. Moreover, the yield and performance of such alloys are reduced, as compared with non-epitaxially grown alloys.
Similar problems are encountered in growing quaternary alloys. Miscibility gaps in the pseudo-quaternary plane and phase separation are the main obstacles for the solidification of quaternary alloys from melts (Lazzari et al., "Growth Limitations by the Miscibility gap in Liquid Phase Epitaxy of Ga.sub.1-x In.sub.x As.sub.y Sb.sub.1-y on GaSb," Mater. Sci. Eng. B, 9:125-128 (1991); Nakajima et al., "The Pseudo-quaternary Phase Diagram of the Ga-In-As-Sb System," J. Crystal Growth, 41:87-92 (1977); and Bachmann et al., "Melt and Solution growth of Bulk Single Crystals of Quaternary III-V Alloys," Progress in Crystal Growth and Characterization, 2:171-206 (1979) ("Bachmann")) and result in alloys which contain substantial defects. These miscibility gaps and phase separation are primarily the result of differences in the chemical interaction between the constituent elements in the melt. To overcome these obstacles, alloys having reduced defects can be prepared by using special solidification techniques, such as the submerged heater method described in Ostrogorsky, "Numerical Simulation of Single Crystal Grown by Submerged Heater Method," J. Crystal Growth, 104:233-238 (1990) ("Ostrogorsky I") and U.S. Pat. No. 5,047,113 to Ostrogorsky ("Ostrogorsky II"). However, even these techniques are not capable of preventing high densities of mechanical cracks related to lattice mismatch. Furthermore, preparation of high quality quaternary alloys using the aforementioned special techniques is expensive and raises the cost of the final product significantly.
For these and other reasons there remains a need for high quality alloys having low incidence of mechanical defects and for methods for preparing such alloys. The present invention is directed to meeting this need.